Secret-communication system



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Nov. 20 1923.

H. A. AFFEL.

SECRET COMMUNICATION SYSTEM Filed July i9, 1922 2 Sheets-Sheet 2 fag NIJMHMQ mwmhm@ n] Midi@ @f7/061' Jin/g m. Y T/ j @Y UNlT HERMAN A. AFFEL, OF BROOKLYN, rNEW YORK, ASSIGNOR T0 AMERICAN TELEPHONE AND TELEGRAEH COMPANY, A CORPORATION OF NEW YORK.

SECRET-COMMUNICATION SYSTEM.

Application led July 19,

To all whom t may cof/teem.'

Be it known that I, HERMAN A. AFFEL, residing at Brooklyn, in the county of Kings and State of New York, have invented certain Improvements in Secret-Communication Systems, of which the following 1s a. specification.

This invention relates to communication systems and more-particularly to arrangements for providing for secrecy in the transmission of messages over such systems.

The arrangements of the invention are particularly applicable lto many of the well known types of radio or carrier telephone and telegraph-systems and cause such systems to so operate that listening in will not be possible with many of .theI relatively simple types of receiving circuits now in use. The operation ofthe arrangements of the invention is based onv a, manipulation of the phase relations of, the two side bands produced in the transmitting circuit by mod ulation and on the provision for the neces- A sary (le-phasing action bythe cooperating receiving station to restore intelligibility.

In radio or carrier systems the natural action of modulation at vthe transmitting station is to provide at thev output of the modulator circuit a carrier frequency and two side-band frequencies, in addition to other non-useful frequency components. While it may be desirable in certain types of carrier systems to suppress, by suitable circuits or filters, the carrier :frequency or one of the side bands, nevertheless in the ordinary type of radios-ystems, as a rule, both side bands, as well as a large proportion of the carrier current, is transmitted. At the receiving station off'such a system, the action'of the two side-bands and the carrier, preserved from the standpoint of phase and amplitude in substantially the same relations as at the ouput of the modu lator circ-uit, provides for the production of the restored voice or signal current. The magnitude of the restored voice or signal current at the receiving station, is a function of the phase relations between the three frequency components involved, due to the fact thatthe restored current is obtained from a combination of two independent aetions; irst, the action of the upper side band and the carrier; second, the action of the lower side band and the carrier. The resultante of both actions must, of course,

1922. Serial No. 575,991.

de-phasing action, or change in the normal phase relation, may be introduced at 'the transmitting end of a radio or carrier. s stem to so disturb the normal relations tween the carrier and side band, that unless means are also provided at the receiving station for restoring the proper phase the volume will be greatly reduced and the message rendered unintelli ible. Such an arrangement would prevent istening in and reception of the signals by the relatively simple types of receiving sets now in use as such sets would ordinarily not be provided with the phase restoring apparatus, nor would they ordinarily provide means of sufticient selectivity for eliminating one of the side bands and thus restoring intell'igibility of the signal. The arrangements of this invention also provide means at the receiving station for restoring the original phase relation between the frequency components to provide an intelligible signal.

It is also Within the intention ofthis invention to shift the phase of one side band to such an extent that the currents transmitted in it will represent substantially different elements of the original voice or signal impulses than are transmitted simultaneously by the other side band, so that the quality will be distorted and rendered unintelligible ir' both side bands are demodulated together in an ordinary receiving station. v

The` invention may be more fully under stood from the following description to gether with the accompanying drawing in the Figures l, 2, 3, 4, 5, and 6 of which are shown radio transmitting and receiving arrangements embodying theprinciples of this invention. Fig. l shows schematically a radio transmitting station including means for shifting the phase of one ofthe transmitted side bands so that the transmitted current will be unintelligible. Fig. 2 shows schematically a radio receiving system conperating with Fig. 1 including means for shifting the phase of one of the received side bands so that an intelligible signal may be received. Figs. 3 and 4 show transmitting and receiving stations respectively which accomplish the above results electrically, while Figs. 5 and 6 show transmitting and receiving stations accomplishing these results mechanically. Like reference characters have been utilized to denote like parts in all of the igures. Vhile the figures of the drawing illustrate transmitting and receivingA stations operating on the principle of radiating energy through lthe ether it is understood that these types of stations might equally well be connected to wire lines of a carrier system.

In Fig. 1 is shown a transmitter T connected to a modulator M. Any well known type of modulator may be employed and hence the modulator is illustrated schematically. The voice or signal currents will be superimposed on a carrier wave at the modulator M. The output of the modulator is connected to a divided circuit, each branch of which includes a filter or selective device, such as F, and F2. These selective devices may be the types of band filters described in the U. S. atent to Gr. A. Campbell, #1,227,113. T e filter F, is designed to readily pass the carrier frequency and the upper side band resulting` from modulation, while the filter F 2 is designed to pass the lower side band. Connected in one of these branch circuits is a phase shifting device PS which will operate to disturb the normal phase relation of one of the side bands, such as the lower side band. The phase shitting device might consist of a filter type network or the filter elements in the`band separating filters F, and F2 might be extended in number :tor one or the other to cause the necessary phase disturbance. The upper and lower side bands and carrier are then recombined and transmitted either byl radiation from the antenna A or over line wires, if desired. It is pointed out that when these components are thus recombined and radiated that the normal phase relations between them are so shifted or disturbed that the message, if received by ordinary apparatus would be unintelligible.

The receiving circuit of Fig. 2 cooperates with the sending station of Fig. 1 and ip cludes the receiving antenna A which is connected to a divided circuit. the branches of which include the band lters F, and F2.

Band filter F1 is designed to select and transmit the carrier and upper side band frequencies and band filter F2 is designed to select and transmit the lower side baud frequency. Associated in one ci these branched selective circuits is a phase shitting device PS', which may be of a type similar to the previously mentioned phase shifting device PS. This phase shitting device will operate to restore the normal phase relation existing between the component fre uencies at the output o modulator M so t at when these transmitted to the demodulator DM an intelligible signal will be received at the receiver R.

In Fig. 3 is shown one modiication of the invention in which secrecy is obtained by shifting one of the side bands through 180C, or, in other words, reversing it in phase from its normal relation to the carrier and the other side band. In this arrangement the voice or signal current from the transmitter T is transmitted over a transmitter circuit which is coupled through a transformer to the circuit of a duplex vacuum bulb modulator M which includes the vacuum bulbs A, and A2. This type of duplex modulator is more fully illustrated in the U. S. patent to J. R. Carson, #1,343,306. In a balanced modulator circuit ot-this type the voice or signal potential applied as shown will render the grid of one ot the tubes positive and the grid oif the other tube negative at the same time. The two output circuits of the modulator are connected to the separate selective circuits 3 and 4 which include the band filters F, and F2. Filter F, will select and transmit the upper side. band from tube A, and filter F2 will select and transmit the lower side band from tube A2. frequency components will not be in phase with each other but one of them will be reversed in phase with respect to the other, due to the manner in which the voice potential affects the grids of the tubes previously pointed out. .Accordingly when these comonents are recombined and transmitted `rom antenna A they will be unintelligible to ordinary receiving apparatus. i

A receiving station adapted to receive, the signals transmitted from an arrangement, such as shown in Fig. 3, is illustrated in Fig. 4. In this arrangement the receiving antenna is associated with the selective circuits including the band tilters F, and F2 to those of Fig. 3, and associated with each of these circuits is a detector circuit, shown as inelud' ing a vacuum bulb detector, as Dl andDz'fOr It is pointed out that these side band the purpose of eliminating the carrierwave.

The output of thc detector circuits are'con-- nected to separate circuits which include the low-pass filters LPF, and LPF2 which serve to eliminate any components ot' the carrier wave and thus give the voice current. The upper and lower conductors 10 and 11 respectively of one of these circuits are connected to the upper and lower conductors of a receiver circuit, while in the other of these circuits the upper conductor 12 is connected 'to the lower conductor of circuit 14 and the lower conductor 13 is connected tothe upper conductor of circuit 14. By connectingv these circuits to circuit 14 in this manner one of the voice or signal currents may again be shifted in phase or reinverted, so that the original phase relation will be restored and when the voice or signal currents `are recombined they will give an intelligible signal.

In Fig. 5 is illustrated an arrangement whereby secrecy is attained by shifting one of the side bands through 180, or reversing it in phase with respect to the other side band. In this arrangement and in the receiving arrangement of Fig. 6 the phase inverting and reinverting is accomplished mechanical means. In the transmitting circuit of Fig. 5 two separate modulator circuits are employed having the modulators M1 and M2, which have been shown as of the duplex vacuum bulbtype. Separate tilter circuits are associated with the output ot each modulator. The filter F3 may be designed to transmit the upper side band while' the lter F, may transmit the lower side band. The modulators may be supplied from a common carrier source S which may also be transmitted through the filter F5 to the antenna A. The separate modulator circuits are excited bythe voice from the same source, as the transmitter T, which, however, is a double' microphone transmitter using a single diaphragm so connected that. while the resistance of one microphone button, such as 6, is increasing, the resistance of the other microphone button, such as 7, is

decreasing. The resultant voice currents applied to the separate modulator circuits will accordin ly be in complete opposition and two side and currents will result with phases oppositely related to the carrier. The secrecy possibilities of such an arrangement reside in the fact. that for reception by ordinary listening in receivin systems, both side bands and carrier wou d fall into one demodulator and no effective voice current. would result.

The receiving arrangements of such a system, shown in Fig. 6, would comprise an antenna A to which would be connected three selective circuits, one for the upper side band including filter F3, 'one for the-lower side band including filter F4. and one for the carrier including filter F5. Separate demodulating circuits having the demodulators DM1 and DM2 would be employed and the carrier selective circuit would be connected in common to each of these. The output of each demodulator would be connected to circuits including the magnets 8 :ind 9 arranged to operate in a push and pull manner on a single diaphragm of receiver R. The voice currents resulting from the side bands would accordingly aid and not oppose each other and intelligible signals would result.

In the above arrangements, phase Shifters are yshown connected in the high frequency circuits and affecting the phase of the side band directly. It will, of course, be understood that the desired phase shift may also he produced by manipulating thc phase` of the voice or signal currents before modulation and at'tcr demodulalion.

lVhile the arrangements ot the invention have been disclosed in certain specific arrangements Awhich are deemed desirable. it is understood that they are capable of embodiment in many and widely, varied forms without departinof from the spirit of the invention as defined in the appended claims.

lVhat is claimed is:

l. In a transmission system in which the frequency components to be transmitted comprise an upper and lower side band resulting from the modulation of a voice or signal current and a carrier current, the method oi' maintaining secrecy in the transmission ot messages in such a system which consists in altering the normal phase relation existing between said components.

2. In a transmission system in which the frequency components to be transmitted coinprise an upper and lower side band resulting from the modulation of a voice or signal current and a carrier current, the method providing secrecy in the transmission ot messages in such a system which consists in altering at the sending station the normal phase relation existing between said com ponents and in restoring at the receiving station the normal phase relation between said components.

3. In a transmission system in which the frequency components to be transmitted conaprise an uiper and lower side band result.- ing from t e modulation ot' a voice or signal current and a carrier current, the method of maintaining secrecy in the transmission of messages in such a system which consists in altering the normal phase relation existing between said components by shifting one of said components in phase through 180.

4. In a transmission system in which the frequencyy components' to be transmitted comprise an upper and lower side band resulting from thc modulation of a voice or siu'nal current. and a carrier current, the method ot' maintaining secrecy in the transmission of messages in such a system which consists in altering the normal phase relation existing between said components by inverting one of said components in phase.

5. In a transmission system in which the frequency con'iponents to be transmitted comprise an upper and lower side band resulting from the modulation of a voice or signal current and a carrier current, the method of providing secrecy in the transmission of messages in such a system which consists in alterinu at the sending station the normal phase reliation existing between said components by inverting one ot said components in phase and in restoring at the receiving station the normal phase relation existing between said components by reinverting said component in phase.

6. In a. secret transmission system a sending station comprising a, talking or signaling circuit, a modulator circuit associated therewith, selective circuits associated with saidv modulator circuit for separating the side band frequencies, and means in one of said selective circuits for altering the normal phase relation existing between said side 10 band frequencies. l

In testunony whereof, I have signed my name to this specification this 17th day of July, 1922. f

HERMAN A. AFFEL. 

